1. Field of the Invention
This invention relates to computer systems and methods for process design, and more particularly to a computer system and method for designing a mixed-model manufacturing process.
2. Description of the Related Art
When designing a manufacturing process, the tasks performed by individual workers and work stations and the physical flows between work stations and materials storage areas must be considered. To make a product, the process design engineer prepares drawings and charts which indicate the various components and subassemblies going into the product as well as the sequence of events that should be followed in combining them. The resulting manufacturing line design is straight forward since a single-model product may be made in a single, static manufacturing line.
The design of a mixed-model manufacturing line, on the other hand, must be consistent with a process technology strategy that will permit making all functional variations (i.e., models) in a family of products. The wide variation in product features and functionality necessitates flexibility in the manufacturing line in order to accommodate the various process changes, customer demanded changes and material variations associated with the variety of products in the family of products. Conventional mixed-model manufacturing lines are organized into fairly autonomous departments grouped by either like-equipment or like-skills in order to produce various components of the products.
To produce each product, the manufacturing line for each product includes a set of processes. Traditionally, mixed-model process lines use batch processing since only a single product "run" is produced at any given time. The path of individual products through various work stations and departments is specified using "routing sheets," which contain the operating steps and routing required for each part, and "process flow charts," which describe the desired sequence of specific tasks (e.g., inspections, movements, and storage operations) for that product.
One problem with conventional mixed-model manufacturing lines is that the variety of products produced by the factory are forced to compete for manufacturing resources and equipment. This competition for resources manifests itself in the batch scheduling where only one product is manufactured at a time. This batch scheduling in turn results in long manufacturing latency, or lead time, for other products. Costs associated with this latency include the opportunity cost of customers unwilling or unable to tolerate the long manufacturing lead times, the costs of re-tooling a machine or work station when switching production from one product to another, and the costs of maintaining sufficient work in process (WIP) at each of the processes to permit filling anticipated product orders.
Customers who are unwilling or unable to tolerate a long manufacturing lead time will look to other sources to fill their demand. Lost customers affect the factory's profitability as they represent lost revenue.
Retooling increases the product's manufacturing cost. In that event, either the cost is absorbed by the factory, thereby reducing unit profit, or else the cost is passed on to the customer in the form of higher selling prices. Unless the product is unique and unavailable elsewhere, a higher selling price usually results in lower demand and, consequently, in lower revenues for that product.
Work-in-process represents a lost opportunity cost. Material and labor in WIP represents tied-up capital that cannot be recovered until processing has been completed and the finished goods (the products) are shipped to the customer.
Manufacturers have developed various manufacturing strategies which seek to mitigate some of the costs associated with mixed-model manufacturing process lines. One strategy, commonly called "just-in-time" (JIT) manufacturing, teaches that just enough work is started to ensure that WIP is minimized. This strategy, however, does not alleviate the batch scheduling inefficiencies of mixed model manufacturing lines, and may result in an excess amount of finished goods inventory. The capital that was invested in WIP is, in JIT processing, invested in finished goods. Other strategies that promise to solve the problems of manufacturing processes, such as "total market quality" (TQM) and the like, suggest changes in the way conventional mixed-model manufacturing lines are managed but do not solve the problems of conventional mixed model manufacturing lines. Accordingly, use of these various strategies has resulted in improvements to the batch-oriented scheduled processing associated with a mixed-model manufacturing line, but the fundamental problems of manufacturing latency, retooling, and unfinished and finished goods inventory remain.
Another problem with conventional mixed-model manufacturing line designs is that dissimilarity in the types of processes used to manufacture the various products introduces resource inefficiencies, such as machine and labor inefficiencies, associated with idle process lines. As discussed above, conventional mixed-model manufacturing is a batch process in which a process group or department works on only one particular product at any given time. Work orders are used to schedule and issue materials required to create the components and subassemblies of that product. These materials are batch-processed into the components and subassemblies, and then staged as WIP until the next process is ready to receive them. During the time the batch process is occurring, the WIP in upstream processes must wait until it can be scheduled into the current process, thereby tying-up capital in the upstream WIP. In addition, downstream processes may be idle while the upstream process completes the batch processing. This idle machine time and the labor and overhead required to maintain the upstream idle process represents an undesirable inefficiency and an additional cost to the factory. The accumulation of WIP and the frequency of idle processes is aggravated when the actual process time (the time to process a standard batch size) at each process is different.
It is thus apparent that there is a need for a system and method for designing a mixed-model manufacturing process line which eliminates manufacturing latencies, and which embodies manufacturing efficiencies and WIP control without incurring a large finished goods inventory. There is also a need for a system and method for designing a mixed-model manufacturing process line which coordinates resources and processes to eliminate idle processes caused by batch-process delays, and delays due to process time imbalances.